docs: fix contributing link (ipfs#457)

(breaks on my linux system as case sensitive)

Author: gilesbradshaw

README.md

@@ -180,7 +180,8 @@ This repository falls under the IPFS [Code of Conduct](https://github.com/ipfs/c
 
 ### Want to hack on IPFS?
 
-[![](https://cdn.rawgit.com/jbenet/contribute-ipfs-gif/master/img/contribute.gif)](https://github.com/ipfs/community/blob/master/contributing.md)
+[![](https://cdn.rawgit.com/jbenet/contribute-ipfs-gif/master/img/contribute.gif)]
+(https://github.com/ipfs/community/blob/master/CONTRIBUTING.md)
 
 ## License